The availability of solid-state image sensors, such as MOS or CCD devices, has renewed the interest in color encoding schemes for cameras having only one image sensor to sense images having a plurality of colors. The inherent geometrical stability of the solid-state sensor allows schemes which practically would be impossible to realize with a pick-up tube, such as a vidicon or saticon. Many color encoding filters have been developed, however, in general these prior art filters have resolution and crosstalk problems which make them unsuitable for use in some high quality, single chip solid-state camera system.
In a frame-transfer CCD (also known as a field-transfer CCD), the whole imaging area is photosensitive. The individual pixels are defined horizontally by vertical channel stops and vertically by horizontal gates having 2, 3 or 4 phase signals applied thereto. As a result of the method of defining the pixels vertically, interlace of the even and odd fields, which cover separate areas in the image for a normal TV signal, is achieved by vertical overlap of pixels in alternate fields. FIG. 1 illustrates a portion of a frame transfer imager 10 with the dotted horizontal lines showing vertical scan boundaries for even fields and the solid horizontal lines showing vertical scan boundaries for odd fields. Scanning line numbers are shown to the left and right of imager 10. A pseudo-interlace is obtained by defining the pixel structure in the two fields with a vertical offset corresponding to one unit of vertical resolution. This mode of operation is equivalent to summing over two units of vertical resolution from adjacent lines where a pixel (picture element) is the combination of two units of vertical resolution in the vertical direction in each field. The vertical resolution limit is not affected by this, but contrast is reduced for vertical spatial frequencies near the Nyquist limit of the vertical sampling.
It should be noted that the present invention is applicable to solid-state devices other than frame-transfer CCDs, for example, to sensors which allow operation with non-overlapping sampling elements such as a MOS diode array sensor. The detailed discussion of the present invention will be devoted to the frame-transfer type device.
The overlapping interlace mode of a frame-transfer CCD which does not allow access to single units of vertical resolution represents a stringent boundary condition for the selection of useful color encoding patterns. For instance, a classical example of a color encoding pattern, the so-called Bayer-pattern, is shown in FIG. 2a, wherein R, G, B refer to red, green, and blue colors respectively, does not work for a frame-transfer CCD, since alternatively only 2 types of signal would be generated, R+G and B+G, and there would be no third type of signal such as G+G. For a full color signal three different signals are required as a minimum.
A whole class of color encoding patterns suited for a frame-transfer CCD are vertical stripe patterns, such as the Yellow-Green-Cyan (Ye, G, Cy) stripes shown in FIG. 2b giving a three color periodicity. The vertical stripe schemes, however, yield relatively poor horizontal resolution, since they require optical low pass filtering to remove aliasing, which removes any spatial frequencies at the stripe filter frequency. For a three pixel period as shown in FIG. 2, the theoretical resolution limit is 2/3 that of the b/w (monochrome) chip. In practice it is even lower, about 50% of b/w resolution.
Resolution can be improved by making use of the second dimension of the image plane for encoding. A class of encoding patterns which does this and still is compatible with a frame-transfer CCD is shown in U.S. Pat. No. 3,982,274 (see FIG. 2c). Here, every second line in the pattern is uniformly colored as indicated by "J" coloration of all pixels on the bottom TV lines of sets 1 and 3 of FIG. 2c, wherein K, L, and J are general colors. As a consequence, the two fields (even and odd) in the video signal have the same colorimetric composition. Lines with elements KJ/LJ and MJ/NJ are generated in both fields, the only difference being that J appears above or below the other element, which is irrelevant for producing the CCD signal. (It may cause flicker under certain circumstances.) A particular pattern of this type is shown in FIG. 2d, wherein w=white or clear. Full resolution in luminance in both directions can be achieved. For chrominance, however, a 1-H delay line is required for decoding and as a consequence, the cameras using such delay lines are rather sensitive to color beats in pictures of objects with certain horizontal line structures. Again, an optical diffuser (two dimensional) is required to help reduce these artifacts.
Referring to FIG. 2e another prior art checkerboard filter pattern for use with single chip solid-state color camera is shown. In this pattern described by Aoki, et al., in a journal article published in IEEE Transactions On Electron Devices, Vol. ED-29, No. 4, April 1982, pp. 745-50 a four color vertical periodicity is provided by a color filter having yellow, green, cyan and white filter elements. In adjacent rows the pattern is shifted by two elements in the horizontal direction such that a cyan element resides vertically between two yellow elements, a white element resides between two green elements, etc. This pattern may provide good performance for an XY-addressed MOS-photodiode sensor where a complete color signal can be derived for each line (without a 1-H delay line) while scanning two rows at a time. However, such a filter pattern is not useful with a device such as a frame-transfer CCD in which only two types of signal would be generated. In fact in the specific example given above the two signals are Ye+Cy and G+W which both yield R+2G+B and no chrominance signal could be generated in a frame transfer CCD using this filter pattern. As described above, with respect to the Bayer-pattern, three different signals are required to reproduce full color.
Another checkerboard filter is described in U.S. Pat. No. 4,288,812 issued on Sept. 8, 1981 in the name of R. N. Rhodes. In this patent the filter elements which overlay less than a pixel area of the imager are shifted from row to row. This filter structure is useful with a CCD frame-transfer device, however, it also requires a 1-h delay line for decoding.
It is, therefore, desirable to provide a checkerboard color filter for use with solid-state imagers, especially for use with single-chip, frame-transfer imagers, that provide the signals necessary to reproduce a color image and that do no require the complexity of a one-horizontal line time delay line. Further, it is desirable to provide signal processing for deriving a full color video signal from a single-chip, solid-state camera.